None.
The present invention relates to a radar altimeter, and more particularly to a precision radar altimeter with terrain feature coordinate location capability.
The precision radar altimeter of the present invention xe2x80x9clooksxe2x80x9d at the ground in a series of swaths, using doppler band pass filters to focus in on one swath at a time. Return signals are received by a pair of antennas. The location of the highest point within a particular swath is determined by performing phase comparisons of the return signals received by the two antennas. If the highest point being illuminated by radar is directly below the air vehicle, then the return signal will come back at the same time to both antennas. On the other hand, if the highest point is off to one side of the air vehicle, the return signal will come back to one antenna before it comes back to the second antenna, because the path is longer for the second antenna. The phase or the time of arrival of the return signals at each of the antennas are compared. The greater the distance between the two antennas, the more accurate the measurement will be. However, as the distance between the two antennas increases, one or more phase ambiguities result.
A phase ambiguity may be understood in the context of a phasor. A phasor repeats every 360 degrees. Therefore, 370 degrees will appear the same as 10 degrees, 380 degrees will appear the same as 20 degrees, and so on. The further apart the two antennas are spaced, the more phase ambiguities will result. Very complex, costly and power consuming ambiguity reducing algorithms are typically incorporated into systems to reduce or eliminate the phase ambiguities. Furthermore, existing systems are xe2x80x9cside-lookingxe2x80x9d, meaning that the antennas for the radar are pointed off to the side of the air vehicle. Side-looking systems process all range cells within a doppler swath, which requires a high level of processing, resulting in large and costly systems. These side-looking radars generate elevation features of the entire area off to the side of the vehicle. These features are correlated with existing electronic terrain elevation maps for navigation purposes. Additionally, the side-pointing antennas must be configured to not illuminate the terrain on the opposite side of the vehicle during roll maneuvers, resulting in rather complex antenna steering mechanisms. The size, weight and cost of existing systems makes it difficult to incorporate the systems on small and medium sized air vehicles.
It would be desirable to use a less complex down-looking radar altimeter system that eliminates phase ambiguities in a more efficient manner, provides the capability to distinguish left targets from right targets, and processes swaths in a more efficient manner by processing only a single range cell within a swath, while maintaining a high degree of accuracy. Furthermore, the capability to distinguish left targets from right targets allows incorporation of standard, low cost, wide beam, radar altimeter antennas. It would also be desirable for a radar altimeter system to improve processing efficiency by processing multiple swaths at one time. A range determination would be calculated for a first swath, while simultaneously performing phase comparisons for a second swath.
A radar altimeter system and method for determining terrain feature location and altitude of an air vehicle comprises a transmitter for transmitting radar signals toward the ground. A first and a second antenna receive reflected radar signals from the ground. A signal processor is coupled to the first and the second antennas. The signal processor includes doppler filter means for rejecting signals other than signals reflected from a selected ground swath. The signal processor determines the above ground level altitude of the air vehicle based on the radar signals output from the filter means. A phase ambiguity resolution means resolves phase ambiguities that arise due to multiple wavelength separation of the first and the second antenna. The signal processor also determines the position of the highest point in the selected ground swath. In a preferred embodiment, the phase ambiguity resolution means comprises a third antenna spaced closely to the first antenna such that there are no phase ambiguities between the reflected radar signals received by the third antenna and the first antenna.
The radar altimeter system of the present invention provides a simplified and efficient means for eliminating phase ambiguities, while maintaining a high degree of accuracy. In addition, processing efficiency is improved because multiple swaths are processed at the same time, and only a single target (the highest target) is processed. In a preferred embodiment, a range determination is calculated for a first swath, while simultaneously performing phase comparisons for a second swath.